Field of the Invention
The present invention relates to an image forming apparatus, which forms an image on a recording medium by using, for instance, an electrophotographic image forming system, such as an electrophotographic copying machine, an electrophotographic printer (for instance, laser beam printer and LED printer) and a facsimile apparatus.
Description of the Related Art
As for a photosensitive member which is used in an electrophotographic type of image forming apparatus, an organic electrophotographic photosensitive member has become prevalent which has a photosensitive layer (organic photosensitive layer) that is made of an organic material as a photoconductive material (charge-generating material and charge-transporting material) provided on a support made from a metal, because of having advantages of low cost and high productivity. As for the organic electrophotographic photosensitive member, an electrophotographic photosensitive member is a mainstream, which has a lamination type photosensitive layer in which a charge-transporting layer that contains a charge-transporting material which is a photoconductive polymer or a photoconductive low-molecular compound is laminated on a charge-generating layer that contains a charge-generating material which is a photoconductive dye or a photoconductive pigment. Thereby, high sensitivity, and diversity of material design is achieved.
To the surface of this photosensitive member, an electric external force and a mechanical external force are applied in an image-forming process. Because of this, durability against the occurrence of scratch and wear on the surface originating in these external forces, in other words, scratch resistance and wear resistance are required. In order to enhance these scratch resistance and wear resistance, in recent years, such a technology is established as to enhance a mechanical strength of the surface layer by using a hardened layer for the surface layer of a photosensitive member. For instance, there is a photosensitive member which includes a hardened layer that is made of a hardening resin as a binder resin, for the surface layer.
However, even though the photosensitive member having the hardened layer as the surface layer is used, the wear of the surface cannot be perfectly prevented. When the hardened layer is worn as the photosensitive drum is used for a long period, the photosensitive layer existing in the lower layer of the hardened layer is exposed, and the wear of the photosensitive layer starts. This photosensitive layer is weak against the mechanical external force, and is rapidly worn from the exposed portion. When the wear of the photosensitive layer which is an insulator progresses, an electric charge results in moving to a support, which is made from a metal and exists in the lower layer of the photosensitive layer, in a worn portion, and the photosensitive layer becomes unable to retain the electric charge. As a result, an image failure occurs in the worn portion.
For this reason, a method for detecting a film thickness of the photosensitive member is conventionally proposed. In Japanese Patent Application Laid-Open No. H05-223513, for instance, a DC current, which flows to the photosensitive member when a voltage is applied to a charging member for charging a photosensitive member to charge the photosensitive member up to a predetermined potential, is detected. A film thickness of the photosensitive member is calculated from the detected current value.
In the configuration of Japanese Patent Application Laid-Open No. H05-223513, the DC current to be detected when the film thickness is detected is generated when the surface of the photosensitive member which is not charged is charged by the charging member. Because of this, the DC current which is detected when the film thickness is detected depends on an area per unit time period of the photosensitive member that enters into a charging portion for charging the photosensitive member. The area of the photosensitive member which enters into the charging portion depends on a rotational speed of the photosensitive member.
Because of this, when the rotational speed of the photosensitive member is, for instance, 200 mm/s which is slower than 400 mm/s, the DC current to be detected becomes small with respect to the film thickness of the photosensitive member, as is illustrated in FIG. 7. If the film thickness decreases as the photosensitive drum is used for a long period, the increase amount of the DC current which is detected becomes small as compared to the decrease amount of the film thickness. In this circumstance, if there is a detection error of the DC current, when the film thickness is calculated, the calculation is easily affected by the error.